Ethernet devices include pulse transformers for the purpose of achieving impedance match and electrical insulation in input/output terminals. Such transformers include magnetic cores generally composed of soft magnetic materials. Such pulse transformers are required to have a high incremental permeability μΔ under the application of a direct-current magnetic field in a temperature range of −40 to 85° C., for example, as defined in American standards ANSI X3.263-1995[R2000]. The incremental permeability μΔ is a value that indicates the degree of magnetization of a magnetic core (core) under the application of a magnetic field.
With a progress in the communication technology in recent years, in Ethernet devices, there has been a trend toward, in addition to a higher transmission speed, a direct supply of driving power of such devices together with transmission signals. In this case, pulse transformers are used under conditions in which large currents may be superposed, compared with conventional conditions. Furthermore, since components around magnetic cores (cores) of pulse transformers within devices generate heat due to such large currents, the temperature of the environment in which the cores are used probably increases.
Accordingly, Mn—Zn ferrite used in such an application is demanded to have a high inductance, that is, a high incremental permeability μΔ, at a higher temperature and under superposition of higher magnetic fields.
A soft magnetic material used in the application is generally Mn—Zn ferrite and various improvements have been proposed.
For example, Patent Literature 1 discloses a technique in which Mn—Zn ferrite is made to contain cobalt oxide to thereby improve magnetic characteristics at a high temperature. However, since Mn—Zn ferrite for magnetic cores of pulse transformers has been designed in terms of composition so as to have a high initial permeability μi, such Mn—Zn ferrite has a low saturation flux density and hence it is difficult to achieve a sufficiently high incremental permeability μΔ at a high temperature and in a high magnetic field.
Patent Literature 2 proposes that reduction in phosphorus and boron are effective to increase incremental permeability μΔ. However, Mn—Zn ferrite disclosed in Patent Literature 2 has a composition selected for the purpose of reduction in core loss and an increase in effective permeability at 100° C. Accordingly, the Mn—Zn ferrite has too low an initial permeability μi at room temperature or less, which is not described in Examples, and hence it is unlikely that the Mn—Zn ferrite has a sufficiently high incremental permeability μΔ in a low-temperature environment.
The above-described impurities are defined in techniques disclosed in Patent Literatures 3 to 6.
Patent Literature 3 proposes a technique in which the content of chlorine is defined to achieve improvements in terms of core loss and amplitude ratio permeability at 100° C. or more. However, it is impossible to make an incremental permeability μΔ at 23° C. be 200 or more by defining the content of chlorine only.
Patent Literature 4 proposes a technique in which the content of sulfur is defined to achieve improvements in terms of power loss. However, it is impossible to make an incremental permeability μΔ at 23° C. be 200 or more by defining the content of sulfur only.
Patent Literature 5 proposes a technique in which the contents of phosphorus, boron, sulfur, and chlorine are defined to suppress exaggerated grain growth in ferrite so that adverse influence on characteristics of ferrite is suppressed. This technique allows a Mn—Zn ferrite having a high resistivity and a small squareness ratio. However, the Mn—Zn ferrite does not have a sufficiently high incremental permeability μΔ in a high magnetic field.
Patent Literature 6 proposes a technique in which the content of phosphorus in a CoO-added ferrite is made very low to suppress exaggerated grain growth in the ferrite so that a high effective permeability is achieved under the application of a direct-current magnetic field. However, since the content of ZnO is high, this technique allows a high incremental permeability μΔ in a low magnetic field of 33 A/m but does not allow a sufficiently high incremental permeability μΔ in a high magnetic field of 80 A/m.
As described above, no existing technique allows a sufficiently high incremental permeability μΔ at a high temperature and in a high magnetic field. To overcome such a problem, the inventors of the present invention developed a Mn—Zn ferrite core having the following configuration and disclosed it in Patent Literature 7:
“A Mn—Zn ferrite core comprising a basic component, sub-components, and unavoidable impurities,
wherein, as the sub-components,
silicon oxide (in terms of SiO2): 50 to 400 mass ppm, and
calcium oxide (in terms of CaO): 50 to 4000 mass ppm; are added to the basic component consisting of
iron oxide (in terms of Fe2O3): 51.0 to 54.5 mol %,
zinc oxide (in terms of ZnO): 8.0 to 12.0 mol %, and
manganese oxide (in terms of MnO): balance and amounts of phosphorus, boron, sulfur, and chlorine in the unavoidable impurities are reduced as follows
phosphorus: less than 3 mass ppm,
boron: less than 3 mass ppm,
sulfur: less than 5 mass ppm, and
chlorine: less than 10 mass ppm.”